John Innes No. 3 is the best one, and it certainly does not need ericaceous anything if, by 'black bamboo', you mean Phyllostachys nigra. But you say there's 'plenty of room in the pot' - does that mean the pot is more than 3 or 4 times bigger than the rootball of the bamboo you have in it? It may be that its been put straight into a pot that's too large for it - it's always best to put in a pot a couple of sizes larger than the rootball, including beneath, and then pot on into larger pots as it grows, till it ends up in the pot you want it to be in.

More info generally in the link below

http://gardenofeaden.blogspot.co.uk/2015/02/how-to-grow-bamboo-in-pots.html

Thanks for advice Bamboo. The larger pot is only 2 sizes bigger so I'll try John Innes.

6" tall? As in bare "fishing poles" cut to 6" tall? Are you sure that it is still alive? A living, tiny bamboo about 6" tall should should be in a pot no more than 8" tall and wide, and will need frequent watering and regular feeding.